Conical micro-structures for super-repellent surfaces and their effect on droplet impact
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Surface wetting properties control is crucial to various applications, such as anti-wetting, self cleaning and heat transfer processes. Design of surface wetting properties can be achieved by mechanical and chemical methods. The mechanical methods include surface roughness design, while chemical methods involve changing the intrinsic wetting properties of the surface. The intrinsic wetting properties, which can be characterized by the contact angle on a flat surface, can reach a maximum of 120°. The combination of roughness and chemical treatment can distinctly enlarge the contact angle range, including wicking (zero contact angle), partial wetting (finite contact angle) and superhydrophobicity (contact angle larger than 150°). Thus, a lot of works showing various types of structure design with different wettability properties can be found in the literature. Among the various possible micro-structures, conical structures (well-known structures found in the Lotus leaf) are frequently used due to their unique properties. The Lotus leaf has a lot of micro-scale tapered bumps and also nano-scale roughness. Various previous works artificially produce this kind of conical structures and achieve similar wetting properties. However, a lot of these works use non-regular conical like structures, while the number of works using patterned conical structured surfaces is limited. Therefore, it remains unclear how the different conical geometries can affect the wetting properties of the surface. To bridge this gap, we fabricate patterned conical micro-structured surfaces with different cones geometry and topography and study how the static and dynamic wetting properties are affected by the structures. The conical micro-structures are produced on silicon substrates using photo-lithography and plasma etching techniques. By varying the fabrication process recipe, different types of conical structures are fabricated. This thesis presents the study of both static and dynamic wetting properties for various conical structured surfaces. In addition, cylindrical pillar structured surfaces are also used for comparison. It is found that the conical structured surface can be designed to be super-repellent for intrinsic contact angles larger than 90°. The conical half-apex angle of the cones is important for suppressing the Cassie-Wenzel transition. This work not only provides more insights into the effect of conical structures on wetting but also shows that conical structures can be a good path for achieving superhydrophobicity. In addition to the previous study of Cassie wetting state on conical structures, we subsequently investigate how the partial wetting Wenzel droplet shape is affected by the conical pillars sidewall geometry. We compare truncated cone pillars with cylindrical pillar surfaces. Previous works show that pillar height/pitch can affect the liquid droplet final shape. However, we observe that the drop shape on truncated cones and on cylindrical pillars is different even when they have the same pitch and height. Besides, the drop shape on these two types of surfaces is also evolving in a different way as the impact Weber number increases. This work reveals that the micro-structures side wall topography can influence the final drop shape. We further investigate conical structures as a means to increase the anti-wetting properties of surfaces during impact of low surface tension droplets. We fabricate conical pillars surfaces with re-entrant like side wall roughness all along the side wall, which looks like a tree-branch topography. Low surface tension drop impact experiments are conducted on these surfaces and we show that the tree-branch like structure does improve the anti-wetting performance by exhibiting a higher critical Weber number (the Weber number starting to show partial rebound), compared with conical structures without a sidewall roughness. The tree-branch like structures can reduce the solid-liquid contact and have higher resistance to penetration, and thus can reach higher anti-wetting performance than other reported rigid surfaces. Finally, we explore how conical and cylindrical pillar structures behave for water droplets impacting at different Weber numbers. We show how the liquid residue size is affected when the droplet impacts above the critical Weber number for conical and cylindrical pillars. It is shown that the conical pillar surfaces have higher contact angle and lower hysteresis while the cylindrical pillars show lower contact angle and higher hysteresis for dense array surfaces. At low Weber number, conical structures surfaces show less energy dissipation compared with cylindrical structures surfaces. For the same height and pitch, the cylindrical pillars show a higher critical Weber number compared with conical pillars due to the large solid-liquid contact at the pillar top. However, the liquid residue when the Weber number is above the critical Weber number for the cylindrical case is larger than for the conical pillar case. We propose that the liquid residue size is affected by the We number, anti-penetration ability and liquid mobility inside the structures. Liquid mobility within the conical structures is lower than for the cylindrical ones, which leads to less wetted area due to less open space inside the structures. This work not only reveals how the conical geometry can affect the wetting properties but also shows that conical structured surfaces are a good candidate for anti-wetting performance enhancement, which can be useful for various applications.
Has partsPaper 1: Ding, Wenwu; Fernandino, Maria; Dorao, Carlos Alberto. Conical micro-structures as a route for achieving super-repellency in surfaces with intrinsic hydrophobic properties. Applied Physics Letters 2019 ;Volum 115.(5) https://doi.org/10.1063/1.5096776
Paper 2: Ding, Wenwu; Dorao, Carlos Alberto; Fernandino, Maria. Anisotropic wetting and final shape of droplets impacting on micropillars with non-vertical lateral walls
Paper 3: Ding, Wenwu; Dorao, Carlos Alberto; Fernandino, Maria. Improving superamphiphobicity by mimicking tree-branch topography.
Paper 4: Ding, Wenwu; Dorao, Carlos Alberto; Fernandino, Maria. Effect of pillar side wall shape on water drop impact behavior.